My girlfriend is seriously paranoid about securing the house before going to sleep every night. While I’m content with a single padlock, she ensures that all locks are in place before turning off the lights. Clearly, when it comes to home security, we are stark opposites.

When it comes to surge protection, however, the roles reverse and I become the paranoid engineer. Regardless of how internally protected from lightning surges the PCB is, I recommend installing external surge arresters. The way I see it, the more surge protectors the merrier.

The Price of Zero Protection

Before addressing the possibility of overprotecting hardware, let’s discuss a scenario where your design lacks any form of surge protection. Despite the popular saying that lightning never strikes twice on the same place, your hardware isn’t safe. Lightning can strike anytime and be close enough to seriously damage your unprotected PCB.

To illustrate how damaging a lightning surge can be, imagine thousands of kilovolts suddenly rushing into the communication port of your PCB. The first component that it hits would burn badly, within mere milliseconds. While the hardware cost may be low, the price for interrupted operations is quite high.

Is it Sufficient to Have Internal Surge Protectors on Your PCB?

When it comes to keeping your PCB safe from damage or destruction, any amount of protection is better than none. The Transient Voltage Suppression (TVS) diode, for instance, is a component that is commonly used to protect electronic components from surges. A TVS diode works by clamping the surge voltage to a specific threshold and diverting excessive surge current to ground.

Another popular transient suppression component is the metal oxide varistor (MOV). A MOV protects electronic components by suppressing the voltage when it exceeds a certain threshold and safely discharging the excessive energy. MOV components are commonly found in power supply units.

Are on-board TVS diodes enough?

The effectiveness of surge protection circuits depends on good component selection as well as the PCB layout techniques involved. For example, a TVS diode needs to be placed near connectors with a shorter trace length than the components it’s meant to protect. This ensures that the transient voltage is not coupled into any adjacent traces.

If designed with best practices in mind, surge suppression circuits can ensure adequate protection for your PCB during some transient events. However, this doesn’t mean that surge protection circuits are a fail-proof solution to lightning surges. Experience has shown that surge protection devices do fail eventually, sometimes after taking just a few large hits.

Is it Worth Paying More For an External Surge Arrester?

When asked why they neglected to install external surge arresters, clients often argue that compared to a few little TVS diodes, external surge arresters can be expensive. In some cases, the cost of a single high-quality external surge arrester can be much higher than the device cost itself.

Despite this complication, there are two convincing reasons to install external surge arresters, even if you’ve already installed on-board protection circuits. The first argument is the amount of testing you have actually conducted to determine the efficiency of the on-board protection circuit. Unless you’ve spent significant time meticulously testing the limits of your on-board protection using simulated transient, you can never be sure how reliable the protection is. On the other hand, external surge arresters are developed by companies who do nothing but perfect transient protection devices.

The second reason that makes installing external surge arresters a great option is that the effect of a failed arrester is minimal. All that is needed is removing the external arrester and replacing it with a new one. This is definitely a much easier process than replacing the entire PCB, which, in most cases, connects to a large number of wires.

It pays to have external protection from surges.

Although placing an external surge arrester is highly recommended, you may choose to rely on internal protectors instead, whether due to cost or time-related limitations. But even if this is the case, you should not neglect to properly design the internal surge protection circuit.

With comprehensive PCB design software like Altium Designer, you can an integrated library to easily place TVS diodes. As an added benefit, you can keep other critical signals away from the edge of the PCB.